The Birth Mother has first rights in Most Countries..if the dad died.."UNLESS" she gave up her parental rights in court legally...as far as I know.. better counsult a lawyer...

When my XH and I drew up our MSA we stipulated if either of us passed that either of our end of the custody arrangement would be taken over by each others family. We have 50/50. You being family can be included but not sure.

Honestly, if I knew then what I know now I wouldn't have set up my MSA that way. I get along fairly well with my XH's wife and family but if something were to happen to him I'd rather take on primary custody and give his family, including his wife, EOW at most and if they wanted to pursue it. I believe taking on the 50/50 custody arrangement would do more harm than good for the kids. I couldn't imagine co-parenting with my ex inlaws or XH's would be widow for that matter.

I doubt BM in your situation would allow the stipulation and if there was one she could still fight it. If BM, or other relative, contests an MSA stipulation or the will there will be factors the courts look into. I.E. how long you've been an influential figure in children lives, how much time your DH (and you) have the children, proximity of children's school district, age of children, if there are other siblings (especially half siblings) in the home.

There was a girl in the neighborhood who's father passed away and her step-mother took her on full-time. A friend of mine has primary custody of her step-daughter from her x-husband. There are cases out there.